CHAPTER 3: DESIGN AND DEVELOPMENT OF THE
3.3 Other CNT X-ray sources
At UNC there are several other X-ray sources based on CNT cathodes. The systems were built for various applications including microbeam radiation therapy7, micro-CT8,
digital chest tomosynthesis9, cell irradiation10, and radiation therapy image guidance11. There are also systems built by XinRay Systems for airport security and rectangular CT12.
In 2006, Kawakita et al. presented a sealed, microfocus X-ray source employing arc- discharge MWNTs on a tungsten wire as the cathode. The tube contained three electrostatic focusing lenses and a copper anode. The tube pressure was 5 × 10-7 torr. A current of 1.3 mA
was produced with 7 kV, imaging a leaf and showing structures of less than 30 µm in size13. Wang et al., in 2008, presented a sealed cathode ray tube with a screen using a “carbon nanometer electron exit” material on a stainless steel rod as the cathode. The tube also contained gate and focusing electrodes. The pressure was 3 × 10-6 torr. The threshold field was 0.95 V/µm, and the maximum current density produced was 1.569 A/cm214.
Beam
Approx. cathode current
(mA)
Dose rate in front (mR/hr)
Dose rate to the right (mR/hr) Dose rate behind (mR/hr) Left- most beam 25.5 0.77 0.49 1.60 Central beam 25.5 0.78 --- 1.98 Right- most beam 25.5 0.76 0.42 1.45
Beginning in 2010, a Korean group began developing CNT X-ray sources. A CNT paste optimized with metal fillers was developed and used in screen-printed field emission cathodes15. Using this cathode, a portable, sealed, and miniature X-ray source was built with a triode configuration. It produced 150 µA, and maintained 10 mA/cm2 for 12 hours15. In 2011, they presented a sealed Kovar and ceramic X-ray tube using the same cathode type, of 10 mm, for the s-DBT application16. The electrode structure was again triode, with a double focusing lens and a molybdenum anode. The cathode was controlled through a high voltage semiconductor switch control, with 2 % fluctuations. Using a 0.3 mm focal spot, a computer mouse was imaged at 30 kVp and 450 V on the gate electrode16. Using the same CNT paste, a pentode configuration microfocus tube was presented in 2012 with a 2 mm × 10 mm cathode containing 0.2 mm CNT dots17. That tube was 71 mm × 35 mm in size, sealed, and made of Kovar and ceramic. This tube employed the same double focus design as the previous tube, with a 0.3 mm focal spot. The anode was tungsten. Again, a computer mouse was imaged with a 2 s exposure at 30 kV. Current of 100 µA was produced at 400 V gate voltage17. Another sealed tube using their high temperature brazing process was presented in 201318. The tube was 6 mm × 38 mm in size, and was triode type. The failure mechanism was studied and determined to be current leakage to the gate. This caused charge buildup on the insulator layer and arcing from the gate to the anode. Without gate leakage, the tube could operate for 250 hours at 25 kVp18.
A sealed, miniature X-ray tube was built in 2012 by Heo et al. The tube measured 10 mm × 50 mm, with further miniaturization planned. A CNT tip on a tungsten wire was the cathode, with a built-in focusing electrode. The anode had a tungsten-film coating, making it
a diode structure. It operated up to 70 kVp and produced 600 µA, equivalent to 0.123 A/cm2. They presented images of a small fish19.
Also in 2012, Ryu et al. presented a triode-configuration tube using a pumped system consisting of a cathode, gate, focus, and anode to be applied to tomosynthesis. The CNT cathode was elliptical and 2.0 mm × 0.5 mm in size. A process for making CNT cathodes called resist-assisted patterning was described where MWNTs were grown on silicon, and then attracted together forming cones. Images of a small animal and electronics were
produced with 3 s exposures at 50 kV and 0.5 mA current. The transmission rate was 70%20. In 2013, Chen et al. presented a sealed microfocus tube with a cathode, gate, and anode. The cathode was made of CNT bundles arrayed on a silicon wafer. Each bundle was 0.1 mm × 0.8 mm and able to produce 2.0 mA, or 2.5 A/cm2. The gate was a copper mesh 250 µm above the cathode. The focusing electrode produced a focal spot size of 35 µm × 39 µm. The anode was copper, covered in tungsten, with a 7° tilt. The tube was able to produce 2 mA cathode current with a TR of 57.5 %, and a gate voltage of 2.7 kV 21.
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